1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly, to a semiconductor device which utilizes a copper wiring layer as a fuse suitable for use with a laser beam having a wavelength in the infrared range or the visible range.
2. Description of Related Art
A semiconductor device including a semiconductor substrate and a plurality of memory cells mounted thereon has conventionally comprised main memory cells and spare cell memories included within the memory cell region for the sake of providing redundancy. In the event that a portion of the memory cell does not operate properly for reasons of mixing of foreign articles into a semiconductor device during the course of manufacture, the backup memory cell is used in place of the inoperative portion (i.e., a defective memory cell) of the memory.
FIG. 9 shows the configuration of the semiconductor device having the spare cell memory. In FIG. 9, reference numeral 31 designates a main memory cell; 32 designates a backup memory cell (hereinafter referred to as xe2x80x9cspare cell memoryxe2x80x9d) provided beforehand in the memory cell region; 33 designates a switching circuit for switching an unillustrated defective memory cell in the main cell 31 to the spare cell memory 32; and 3 designates a fuse for programming the switching circuit 33 through opening. First, the address of the defective memory cell in the main cell 31 is detected through use of test equipment. The fuse 3 connected to the switching circuit 33 is opened by means of a laser beam, thereby programming the switching circuit 33 so as to substitute the spare cell memory 32 for the defective memory cell. More specifically, a word line, or the like, connected to the defective memory cell is disconnected, and the spare cell memory 32 is arranged so as to be selected when the address of the defective memory cell (i.e., a defective address) is selected.
The structure of the fuse 3 and the principle according to which the fuse layer 3 is opened by a laser beam will now be described by reference to FIGS. 10 through 13.
FIG. 10 is a plan view of the fuse 3. Opposite ends of the fuse 3 are connected to the interior of the respective switching circuits 33. The fuse 3 is opened by irradiation of its center portion 10 with a laser beam (radiated from above; i.e., perpendicular to, the sheet of FIG. 10). FIG. 11 is a cross-sectional view of the fuse 3 in its transverse direction (along line A1-A2 shown in FIG. 10). In FIG. 11, the fuse layer 3 is formed on a silicon substrate 1 by way of a first dielectric film 2. The fuse layer 3 comprises a barrier metal layer 3a and a copper wiring layer 3b formed thereon. A second dielectric film 5 is formed on the fuse layer 3. When the fuse layer 3 is exposed to a laser beam 6, the laser beam 6 is absorbed by the fuse layer 3, which consequently increases in temperature. As a result, the fuse layer 3 is changed in phase from a solid to a liquid and further to a vapor. As shown in FIG. 12, as a result of the changes in phase of the fuse layer 3, the bottom of the second dielectric film 5 is raised by the vapor pressure stemming from evaporation of the fuse layer 3, thereby forming a space 12. As shown in FIG. 13, when the vapor pressure within the space 12 exceeds a predetermined value, the fuse layer 3 is opened, the area of the second dielectric film 5 situated above the fuse layer 3 is blown, thus forming a blow pocket 15. If no second dielectric film 5 used for forming the space 12 for the purpose of withstanding a predetermined vapor pressure is provided, the fuse layer 3 is merely fused. Consequently, opening the fuse under the foregoing method involves the second dielectric film 5 used for blowing the fuse layer 3. A desirable thickness of the second dielectric film 5 is 0.4 to 1.0 xcexcm or thereabouts.
A laser of infrared wavelength has conventionally been used to generate the laser beam 6 that is used for opening the fuse layer 3, in consideration of sufficient absorption of light by the fuse layer 3 and preventing the laser 6 from damaging the silicon substrate 1 underlying the fuse layer 3. For example, a Yttrium Lithium Fluoride (YLiF4) YLF laser having a wavelength of 1.047 xcexcm or 1.321 xcexcm is widely used.
Recently, an increase in the number of layers of elements constituting a semiconductor device and an increase in film thickness stemming from a reduction in the number of processes for manufacturing a semiconductor device render more difficult disconnection of the fuse layer that is provided as a lower layer. For this reason, adoption of a wiring layer which is positioned at the highest possible_position has recently become popular.
A metal wiring layer, for example, a copper wiring layer, is adopted as an upper wiring layer. An explanation is now given of a case where the copper wiring layer is used as a fuse layer.
FIG. 14 is a graph showing the spectral light-absorption characteristic of copper and aluminum, wherein the vertical axis represents a reflectivity R(%) and the horizontal axis represents a wavelength xcex(xcexcm). As shown in FIG. 14, the infrared wavelength 42 generally designates a wavelength xcex of more than 0.76 xcexcm, and the visible wavelength 41 generally designates a wavelength xcex ranging from 0.38 to 0.761 xcexcm. A laser beam of the infrared wavelength 42 has high reflectivity R with respect to copper present in a copper wiring layer forming a fuse layer. Consequently, a light absorption coefficient for copper (=1xe2x88x92R) is as low as several percent. Thus, the fuse cannot sufficiently absorb the laser and is considered to be difficult to open. Copper has a low reflectivity R relative to the visible wavelength 41, particularly light whose wavelength is shorter than a green wavelength of 0.57 xcexcm. Therefore, copper has a high light absorption coefficient for visible light. Further, the silicon substrate 1 underlying the copper fuse has also a high light absorption coefficient for the laser of visible wavelength 41, relative to that for the laser of infrared wavelength 42. In a case where the copper fuse is opened through use of a laser beam of visible wavelength 41 whose absorption factor for copper is large, a laser beam of visible wavelength 41 radiated to open the copper fuse damages the silicon substrate 1.
As mentioned previously, when the copper fuse is opened through use of the laser beam of visible wavelength whose absorption coefficient for copper is large, the laser beam of visible wavelength radiated so as to open the copper fuse damages the silicon substrate underlying the copper fuse.
The present invention has been conceived to solve the foregoing problem, and the object of the present invention is to provide a semiconductor device having a fuse whose structure mitigates damage to the silicon substrate underlying the copper fuse through exposure of the copper fuse to a laser beam.
According to a first aspect of the present invention, there is provided a semiconductor device comprising: a semiconductor substrate; a first dielectric film formed on the semiconductor substrate; and a second dielectric film formed on the first dielectric film, wherein the first dielectric film has a fuse layer in a part of the area on the first dielectric film, the fuse layer including a copper wiring layer and the second dielectric film has a light absorbing layer located on the fuse layer, and the light absorbing layer is formed such that the absolute value of a real number term of a complex dielectric constant of the light absorbing layer is smaller than the absolute value of a real number term of a complex dielectric constant of the copper wiring layer and such that an imaginary number term of the complex dielectric constant of the light absorbing layer is greater than an imaginary number term of a complex dielectric constant of the copper wiring layer.
According to a second aspect of the present invention, there is provided a semiconductor device comprising: a semiconductor substrate; a first dielectric film formed on the semiconductor substrate; and a second dielectric film formed on the first dielectric film, wherein the second dielectric film includes a first wiring layer in a portion of the second dielectric film, the first wiring layer having a predetermined light absorption coefficient and the first dielectric film has a second wiring layer located beneath the first wiring layer, the second wiring layer having a light absorption coefficient which is not greater than a light absorption coefficient of the first wiring layer.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of the embodiments thereof taken in conjunction with the accompanying drawings.